This invention relates to method and apparatus for forming thermoplastic foams. More particularly, it relates to an improved method of forming thermoplastic foams having enlarged cross-sectional areas, i.e., greater than those of the foams allowed to foam freely, and a new apparatus used in connection with such method.
Foams of thermoplastic resins such as, for example, polystyrene and polyethylene, are useful industrial products because of their excellent heat-insulating, cushioning and other properties. These foams have found acceptance over the years in such applications as thermal insulation and cushioning as well as raw material for the fabrication of various shaped articles. The preparation of thermoplastic foams by extruding a heat-plastified mixture of a thermoplastic resin and a blowing agent is well known in the art and is described in U.S. Pat. Nos. 2,740,157; 3,067,147; 3,413,387; 3,413,388; 3,431,163; 3,431,164; 3,954,929 and 3,966,381 and Canadian Pat. No. 451,864.
In many end use applications, it is desirable to obtain thermoplastic foams having large cross-sectional areas. In order to obtain such foams, it has been necessary to use dies having relatively large extrusion orifices with corresponding increases in extrusion output capacities. Such practice requires frequent die changes which are expensive and time consuming. Furthermore, it has also been necessary to inventory a large number of dies having different dimensions of extrusion orifice with the attendant increase in the cost of equipment and storage space.
It is especially desirable to obtain thermoplastic foams having relatively large cross-sectional areas of generally rectangular cross-sectional configurations. In order to obtain such foams, several methods have been used. In one of such methods, a thermoplastic foam having a cross-sectional configuration closely approximating the configuration of a rectangle is extruded and subsequently shaped into a rectangular configuration by trimming off the edges thereof. Such trimming results in a waste of material, equipment and labor. Sometimes, it is necessary to adhesively laminate several layers of relatively thin thermoplastic foams together to realize the desired thickness for a given foam. Such a step requires an additional operating step involving costly equipment and further labor input. In order to overcome these disadvantages, several approaches have been suggested. U.S. Pat. No. 3,966,381, for example, teaches a method of directly extruding thermoplastic foams having generally rectangular cross-sectional configurations: A heat-plastified foamable gel is extruded through an elongate slit-like extrusion orifice into a region defined by a pair of spaced apart, generally parallel shaping plates, lying generally parallel to and symmetrical about the plane of the axis of extrusion and the major axis of the die opening, and a pair of generally concave restraining means (edge restraining means) positioned adjacent to the ends of the die, and the gel is cooled to a self-supporting foam. The restraining means constrain the freshly-extruded gel in generally convex configuration in the plane of the axis of extrusion and the major axis of the die opening at a location adjacent to the die. The restraining means having a generally concave surface in a plane perpendicular to the axis of extrusion engages edges of the gel and applies friction thereto to provide the resulting thermoplastic foam with a generally rectangular cross-sectional configuration. All surfaces which contact the freshly-extruded foam are coated with polytetrafluoroethylene to prevent sticking of the foam thereto. However, recent attempts to make a polyethylene foam, in accordance with the teachings of U.S. Patent No. 3,966,381, have resulted in repeated failures. More specifically, the foam tears because of high friction between the surfaces thereof and those of the polytetrafluoroethylene coating on the metallic shaping plates. Even with foams having relatively low friction, such as, for example, foams of polystyrene, the skin quality thereof needs further improvement. Furthermore, the curved restraining means is generally time consuming and expensive to make.
This invention provides an improved method of forming thermoplastic foams having enlarged cross-sectional areas of various geometric configurations and a new apparatus in connection with such method. More particularly, the invention provides thermoplastic foams having cross-sectional areas significantly greater than those obtained without any aid of forming, i.e., free foaming, and having generally rectangular cross-sectional configurations. The thermoplastic foams produced in accordance with this invention generally have improved skin quality and improved compressive strength in the vertical direction. In one aspect, this invention also provides thermoplastic foams generally having more straight rectangular edges thereon which are obtained using a relatively inexpensive edge restraining means.